The present invention relates to an optical pulse generator which can selectively change a wavelength of an optical pulse signal.
In a general-use optical pulse generator, an optical fibre made of a predetermined material added with a rare-earth material is vibrated by a displacement element such as a piezoelectric element so as to open and close an optical path provided for an optical signal, so that an optical pulse signal is generated.
FIG. 3 is a drawing showing an electric configuration of an example of the optical pulse generator. In FIG. 3, a numeral 1 designates a signal source having a wavelength .lambda..sub.a,while a numeral 2 designates an excitation light source having another wavelength .lambda..sub.b ; 3 designates an optical fibre added with the rare-earth material; 4 designates an optical switch; 5 designates a wave divider; 7 designates a wave divider/mixer; 8A and 8E designate lenses; and 9 designates a slit plate. In the optical pulse generator shown in FIG. 3, a predetermined wavelength is selected for a signal beam by the wave divider 5, and then, an amplitude of the pulse-like signal beam is enlarged (in other words, the optical pulse signal is amplified).
In general, the optical pulse is defined as a light which is radiated for a short period of time. This optical pulse has a pulse like waveshape having a peak level. Normally, a pulse width for the optical pulse is defined as a period of time in which an amplitude level of the optical pulse is approximately higher than a half of the peak level. Thus, the waveshape of the optical pulse can be defined by use of the peak level and the pulse width.
Now, the wave divider S removes unnecessary wavelength components from the signal beam produced from the signal source 1 so as to only pass a signal component having the wavelength .lambda..sub.a therethrough. On the other hand the wave divider/mixer 7 introduces the excitation beam having the wavelength .lambda..sub.b, produced from the excitation light source 2, to the optical fibre 3 so as to excite the optical fibre 3. By driving the optical switch 4. The optical path provided between the signal source 1 and the optical fibre 3 is opened for a short period of time. In such short period of time. The signal beam outputted from the signal source 1 is introduced into the optical fibre S wherein it is amplified. Then, the signal beam amplified is passed through the wave divider/mixer 7 from which it is outputted as an output beam.
Next, a configuration and an operation of the optical switch 4 will be described in detail by referring to FIG. 4. In FIG. 4. a numeral 1A designates an optical fibre which introduces the signal beam outputted from the signal source 1; 3A designates a terminal portion of the aforementioned optical fibre S made of the predetermined material including the rare-earth material; 4A designates a displacement element; 4B designates a fulcrum point; 4C designates a transmission plate; and 4D designates a drive circuit. These elements 4A to 4D are assembled together to form the optical switch 4.
The displacement element 4A is vibrated when being driven. As the displacement element 4A, it is possible to employ an piezoelectric element, a tuning-fork vibrator, a quartz oscillator and the like. The terminal portion 3A of the optical fibre 3 is formed as a free terminal, while the optical fibre 3 itself is supported at the fulcrum point 4B. The transmission plate 4C transmits a vibration caused by the displacement element 4A to a certain part of the optical fibre 3 which is existed between the terminal portion 3A and the fulcrum point 4B.
In FIG. 4. The optical fibre 4 is located to face with the terminal portion 3A of the optical fibre S. Then, by vibrating the optical fibre 3. An optical path between the optical fibres 1A and 3 is opened or closed.
Next, a vibrating state of the terminal portion 3A of the optical fibre 3 will be described by referring to FIG. 5. For example a distance between the terminal portion 3A and the fulcrum point 4B is set at 1 cm. It can be observed from FIG. 5 that a vibrating pitch of the terminal portion 3A of the optical fibre 3 is larger than that of the transmission plate 4C. If a core diameter of the optical fibre 1A is 10 .mu.m, by vibrating the terminal portion 3A of the optical fibre 3 with a vibrating pitch of 10.mu.m or more, it is possible to open and close the optical path between the optical fibres 1A and 3.
Next a construction of the slit plate 9 shown in FIG. 3 will be described in detail by referring to FIG. 6. Under beams are converged on a crossing point at which location the slit plate 0 having a slit 9A is located. For example, an opening interval of the slit 9A is set at 10.mu.m.
The lens 8E and the optical fibre 3 are arranged such that when the displacement of the displacement element 4A is equal to zero. The lens 8E is located to be connected with the optical fibre 3. The slit plate 0 shuts off any light components other than the signal beam. As the signal beam, a pulse-like beam or a continuous beam can be employed if its width is larger than a pulse width of the signal beam to be amplified.
When the displacement is equal to zero, an amplitude of the signal beam (i.e.. optical pulse signal) reaches the peak; while as the displacement becomes larger. The amplitude of the signal beam becomes lower. If a pulse width of the signal beam to be inputted into the optical fibre 3 is smaller than an optical-connection period in which lens 8E and the terminal portion 3A is optically connected it is difficult to control the pulse width (or wavelength) of the signal beam to be outputted. However if the pulse width of the signal beam is larger than the above optical-connection period, it is possible to control the pulse width of the signal beam to be outputted from the optical fibre 3. For this reason, it is necessary to select the pulse width of the signal beam inputted larger than the above optical-connection period. Thus it is possible to employ the continuous light as the signal beam inputted because its pulse width is very large.
Meanwhile the pulse width of the signal beam to be amplified depends on a time at which the displacement of the displacement element 4A is set at zero. Because, as described before when the displacement of the displacement element 4A is zero. The amplitude of the signal beam outputted reaches the peak. While the pulse width of the signal beam is determined responsive to the amplitude level.
Incidentally the drawings shown in FIGS. 3, 4, 5 and 6 are identical to FIGS. 6, 1, 2 and 3 attached with the specification of the Japanese Patent Application No. 4-179242 whose filing date is Jun. 12, 1992. Of course, this application has not been published in Japan.
By the way, the wave divider 5 shown in FIG. 3 is designed to selectively pass the signal beam having a specific wavelength only. Thus, there is a problem in that the wavelength of the signal beam outputted cannot be arbitrarily altered.